Among the battery which is practically used, the lithium ion secondary battery exhibits the highest energy density, thus it is particularly used for the compact electronics. Also, in addition to the compact use, it is also expected to be used for the automobiles. Under such situation, it is in demand to make the lifetime of the lithium ion secondary battery longer and to have higher capacity, and to further improve the safety.
In the lithium ion secondary battery, generally in order to prevent the short circuit between the positive electrode and the negative electrode, polyolefin based organic separator such as polyethylene or polypropylene or so is used. The polyolefin based organic separator has a physical property which dissolves at 200° C. or less, thus in case the temperature of the inside of the battery becomes high due to the stimulus from the outside and/or the inside; shrinking or melting may occur, which causes to change the volume. As a result, the short circuit of the positive electrode and the negative electrode, and the explosion due to the release of the electrical energy or so may happen.
Hence, in order to solve such problems, conventionally it is proposed to form the porous membrane (hereinafter it may be referred as “porous membrane”) which includes non-conductive particle such as in organic particle on the electrodes or the separators.
For example, Patent document 1 discloses the porous membrane comprising plate shape particles as the insulating fine particles and the binder having the rupture elongation of 300% or more. According to Patent document 1, this porous membrane is arranged in a certain direction, thus it has better safety than using the spherical particles. Further, it is disclosed that it has excellent porous membrane elongation since the binder having the rupture elongation of 300% or more is used.